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<title>14.14.1.7 Specifying the required argument types (function prototypes)</title>
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<h3><a name="SECTION00161417000000000000000"></a><a name="ctypes-specifying-required-argument-types"></a>
<br>
14.14.1.7 Specifying the required argument types (function prototypes)
</h3>

<p>
It is possible to specify the required argument types of functions
exported from DLLs by setting the <tt class="member">argtypes</tt> attribute.

<p>
<tt class="member">argtypes</tt> must be a sequence of C data types (the <code>printf</code>
function is probably not a good example here, because it takes a
variable number and different types of parameters depending on the
format string, on the other hand this is quite handy to experiment
with this feature):
<div class="verbatim"><pre>
&gt;&gt;&gt; printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
&gt;&gt;&gt; printf("String '%s', Int %d, Double %f\n", "Hi", 10, 2.2)
String 'Hi', Int 10, Double 2.200000
37
&gt;&gt;&gt;
</pre></div>

<p>
Specifying a format protects against incompatible argument types (just
as a prototype for a C function), and tries to convert the arguments
to valid types:
<div class="verbatim"><pre>
&gt;&gt;&gt; printf("%d %d %d", 1, 2, 3)
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: wrong type
&gt;&gt;&gt; printf("%s %d %f", "X", 2, 3)
X 2 3.00000012
12
&gt;&gt;&gt;
</pre></div>

<p>
If you have defined your own classes which you pass to function calls,
you have to implement a <tt class="method">from_param</tt> class method for them to be
able to use them in the <tt class="member">argtypes</tt> sequence. The <tt class="method">from_param</tt>
class method receives the Python object passed to the function call,
it should do a typecheck or whatever is needed to make sure this
object is acceptable, and then return the object itself, it's
<tt class="member">_as_parameter_</tt> attribute, or whatever you want to pass as the C
function argument in this case. Again, the result should be an
integer, string, unicode, a <code>ctypes</code> instance, or something having
the <tt class="member">_as_parameter_</tt> attribute.

<p>

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